In today's rapidly-advancing optical electronics industry, vertical cavity surface emitting lasers have become preferred as the optical source. Vertical cavity surface emitting lasers—also referred to as VCSELs—are favored because of the ease of their manufacture, the repeatability of the manufacturing process used to form the VCSELs, the reduced substrate area each VCSEL requires, and because of the superior uniformity of the VCSELs formed within the same substrate. Furthermore, vertical cavity surface emitting lasers typically require less power to drive their lasing action than edge emitting lasers. A principal characteristic of a VCSEL is that it emits beams vertically, i.e. in a direction normal to the P-N junction and the surface of the semi-conductor substrate on which it is fabricated. There are at least two issues, however, associated with the use of VCSELs in optoelectronic systems.
One issue is monitoring the optical output of the VCSEL. In conventional edge emitting lasers, one end of the laser serves as the emitting edge, while the opposite end may be used to monitor optical power once the relative amount of light emitted out of the respective ends is determined. A small portion of light is typically emitted out of the end that is not used as a primary optical source. Most commercially available VCSELs emit light normal to the surface in which they are formed. Therefore, in order to monitor optical power, this emitted beam must be monitored. It is challenging to do this without blocking or otherwise obstructing the optical beam, which must also be focused onto an optical transmission medium. It is thus desirable to provide a detector that monitors the emitted optical beam without attenuating or compromising it.
Another issue associated with the use of VCSELs is that the light emitted from a VCSEL mounted on a module according to conventional techniques, is normal to the fiber launch direction used in most optical communication applications. Fiber-connected optoelectronics in high-speed applications typically require that light is advantageously emitted and received parallel to the plane of the module such as the surface of a printed circuit board. The launch direction of the optical fiber, along which light travels, is also preferably parallel to the plane of the module. In this manner, the light is emitted and received along a direction generally parallel to the path of the electric signal. It is therefore a challenge to mount a VCSEL within an optical subassembly mounted on a printed circuit board and which will be coupled to an optical fiber oriented generally parallel to the printed circuit board. When using vertically-transmitting optical devices such as VCSELs, either the electrical or optical path must make a 90° turn in order to achieve parallel connection with the fiber according to conventional packaging technologies. Mirrors may be used to bend the light 90° to try to focus the emitted light onto the end face of a fiber without compromising the quality of the optical signal. Even if the VCSEL is mounted such that it is rotated 90° with respect to the printed circuit board, the stability of the optical subassembly (OSA) mounted sideways on the board becomes a concern, and the nature and length of the electrical connections between the OSA and the board also becomes a concern, especially in high-frequency applications where a constant and controlled impedance is typically required. Moreover, there are space constraints in many applications that limit OSA designs, and therefore the ability to mount a vertically-emitting optical device within an OSA and perpendicular to the module such that it emits light parallel to the plane of the module. Any such space constraints associated with mounting an OSA on a printed circuit board mandate that the OSA be of minimal dimension, which may make it difficult to utilize OSAs large enough to include additional components capable of turning the optical path. Similar shortcomings and challenges may be present for mounting vertically-receiving optical devices as well.
The cost of an OSA generally increases with the number of components which combine to form the OSA. Such components typically include a separately formed and assembled ball lens to focus the light emitted from a laser into the end face of an optical fiber. It would therefore be desirable to reduce cost by eliminating components such as the ball lens.
What is needed to address the various shortcomings of the conventional technology, is a method and apparatus for mounting a vertically-emitting or receiving optical element in an optical subassembly such that the optical element is oriented to emit or receive light along a fiber launch direction that is parallel to the surface of the module on which the optical subassembly is mounted.